Re: [PATCH V4 2/9] virtio_ring: switch to use dma_{map|unmap}_page()

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Fri, 18 Jul 2025 17:16:09 +0800, Jason Wang <jasowang@redhat.com> wrote:
> This patch switches to use dma_{map|unmap}_page() to reduce the
> coverage of DMA operations. This would help for the following rework
> on the virtio map operations.
>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/virtio/virtio_ring.c | 55 +++++++++++++++---------------------
>  1 file changed, 23 insertions(+), 32 deletions(-)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 103bad8cffff..75e5f6336c8d 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -405,8 +405,8 @@ static dma_addr_t vring_map_single(const struct vring_virtqueue *vq,
>  	if (!vq->use_dma_api)
>  		return (dma_addr_t)virt_to_phys(cpu_addr);
>
> -	return dma_map_single(vring_dma_dev(vq),
> -			      cpu_addr, size, direction);
> +	return virtqueue_dma_map_single_attrs(&vq->vq, cpu_addr,
> +					      size, direction, 0);
>  }
>
>  static int vring_mapping_error(const struct vring_virtqueue *vq,
> @@ -451,22 +451,14 @@ static unsigned int vring_unmap_one_split(const struct vring_virtqueue *vq,
>  	if (flags & VRING_DESC_F_INDIRECT) {
>  		if (!vq->use_dma_api)
>  			goto out;
> +	} else if (!vring_need_unmap_buffer(vq, extra))
> +		goto out;
>
> -		dma_unmap_single(vring_dma_dev(vq),
> -				 extra->addr,
> -				 extra->len,
> -				 (flags & VRING_DESC_F_WRITE) ?
> -				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -	} else {
> -		if (!vring_need_unmap_buffer(vq, extra))
> -			goto out;
> -
> -		dma_unmap_page(vring_dma_dev(vq),
> -			       extra->addr,
> -			       extra->len,
> -			       (flags & VRING_DESC_F_WRITE) ?
> -			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -	}
> +	dma_unmap_page(vring_dma_dev(vq),
> +		       extra->addr,
> +		       extra->len,
> +		       (flags & VRING_DESC_F_WRITE) ?
> +		       DMA_FROM_DEVICE : DMA_TO_DEVICE);
>
>  out:
>  	return extra->next;
> @@ -1276,20 +1268,13 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
>  	if (flags & VRING_DESC_F_INDIRECT) {
>  		if (!vq->use_dma_api)
>  			return;
> +	} else if (!vring_need_unmap_buffer(vq, extra))
> +		return;
>
> -		dma_unmap_single(vring_dma_dev(vq),
> -				 extra->addr, extra->len,
> -				 (flags & VRING_DESC_F_WRITE) ?
> -				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -	} else {
> -		if (!vring_need_unmap_buffer(vq, extra))
> -			return;
> -
> -		dma_unmap_page(vring_dma_dev(vq),
> -			       extra->addr, extra->len,
> -			       (flags & VRING_DESC_F_WRITE) ?
> -			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> -	}
> +	dma_unmap_page(vring_dma_dev(vq),
> +		       extra->addr, extra->len,
> +		       (flags & VRING_DESC_F_WRITE) ?
> +		       DMA_FROM_DEVICE : DMA_TO_DEVICE);
>  }
>
>  static struct vring_packed_desc *alloc_indirect_packed(unsigned int total_sg,
> @@ -3161,7 +3146,13 @@ dma_addr_t virtqueue_dma_map_single_attrs(const struct virtqueue *_vq, void *ptr
>  		return (dma_addr_t)virt_to_phys(ptr);
>  	}
>
> -	return dma_map_single_attrs(vring_dma_dev(vq), ptr, size, dir, attrs);
> +	/* DMA must never operate on areas that might be remapped. */
> +	if (dev_WARN_ONCE(&_vq->vdev->dev, is_vmalloc_addr(ptr),
> +			  "rejecting DMA map of vmalloc memory\n"))
> +		return DMA_MAPPING_ERROR;


Why add this check, we never use the vmalloc address?

Thanks.


> +
> +	return dma_map_page_attrs(vring_dma_dev(vq), virt_to_page(ptr),
> +				  offset_in_page(ptr), size, dir, attrs);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_dma_map_single_attrs);
>
> @@ -3186,7 +3177,7 @@ void virtqueue_dma_unmap_single_attrs(const struct virtqueue *_vq,
>  	if (!vq->use_dma_api)
>  		return;
>
> -	dma_unmap_single_attrs(vring_dma_dev(vq), addr, size, dir, attrs);
> +	dma_unmap_page_attrs(vring_dma_dev(vq), addr, size, dir, attrs);
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_dma_unmap_single_attrs);
>
> --
> 2.47.3
>